Wei-Hwa Huang's website was what I originally used to learn, but as one would expect there are some errors and it's very out of date. It's still a good reference site, though.
As for Yakitori Online, it went down because the owner wasn't getting loads of cash from would-be Japanese mahjong players.
It wasn't a very helpful website either, partly because most of the "meets" were conducted at ron2. I only attended one of those "meets"...and ended up winning.
